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Plumber's solder is composed of 67% lead and 33% tin by weight.

Take a look at the phase diagram for a tin-lead mixture in the graph below. By weight, the solder combination contains 67 percent lead and 23 percent tin. If the mixture cools, the temperature will drop from the dashed red line on the graph until it hits the temperature boundary between the liquid phase and the solid Pb-Liquid phase, which is 250 K, and the lead will begin to freeze. The remaining liquid includes a higher percentage of tin, so the point at which it freezes will be lower, and the remaining liquid will again contain a higher percentage of tin, so the point at which it freezes will be lower. This cycle will continue until the temperature drops below freezing. Because the two curves of the two borders will meet at this moment, all of the remaining mixture will freeze (this point called the eutectic point).

The melting temperature of the mixture will decrease more than the eutectic composition for joining the pipes in our situation (where the solder mixture contains 67 percent lead and 23 percent tin by weight). The mixture is more suitable for soldering if the melting temperature is lower. As a result, the tin and lead mixture is better for joining pipes.